This application claims the priority of Korean Patent Application No. 2002-87942, filed on Dec. 31, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical lens, and more particularly, to a hybrid achromatic optical lens whose chromatic aberration is corrected in order to be used in an optical data storage system.
2. Description of the Related Art
Optical data media have evolved from compact discs (CDs) having a diameter of 12 cm and a substrate thickness of 1.1 mm to digital versatile disc (DVDs) having a diameter of 12 cm and a substrate thickness of 0.6 mm. Recently, blu-lay discs thinner than the DVDs have been developed. In order to keep pace with the advancements of optical data media, optical data storage systems for recording data on the optical data media and reproducing the recorded data have been developed so as to form a small light spot having a high energy on the optical data media and obtain a high recording density. For example, an optical data storage system for a CD uses a light source having a wavelength of 780 nm and a lens having a numerical aperture (NA) of 0.45, whereas an optical data storage system for a DVD uses a light source having a wavelength of 680 nm and a lens having a numerical aperture of 0.6.
Optical data storage systems stably record and reproduce data by automatically adjusting variations in a distance between an optical data medium and a lens according to the motion of the optical data medium using a servo mechanism. However, the servo mechanism cannot compensate for a mode hopping phenomenon of a laser beam, in which the wavelength (unit: nanometer) of the laser beam is shifted according to changes in the operational temperature. In the mode hopping phenomenon, a spectrum instantaneously hops to another mode according to changes in the operational temperature. In addition, when the mode hopping phenomenon occurs, a plurality of modes overlap so that the direction of the modes instantaneously changes in a forward or a reverse direction, thereby resulting in a rapid change in the laser wavelength with the output power and generation of noise.
In order to solve the above problems, a method of using a light source emitting a laser beam having a stable frequency and a method of compensating for chromatic aberration occurring in a longitudinal direction of a frequency band of a laser beam and removing noise can be used. Here, chromatic aberration describes a situation where the wavelength of light passing through a refractive lens changes according to changes in temperature and the light is focused onto different positions in a direction of an optical axis of the light.
Generally, the light source for emitting the laser beam having the stable frequency is expensive, and thus the method of compensating for chromatic aberration using the lens is mainly used. The method of compensating for chromatic aberration includes a first method of using a lens combining a conventional convex lens and a conventional concave lens, a second method of using a gradient index (GRIN) lens whose index of refraction changes in an axis direction and/or a radial direction thereof, and a third method of using a lens combining a diffractive element and a refractive element. The first method is not suitable for the optical data storage systems because the weight and the size of the combination lens are great. The second method is disadvantageous in that the method for manufacturing the GRIN lens is complex.
FIG. 1 shows a hybrid lens 10 for compensating for chromatic aberration disclosed in U.S. Pat. No. 5,349,471. The hybrid diffractive/refractive lens 10 for use in an optical data storage system includes an optical disc substrate 14 made of transparent plastic, such as polycarbonate. The lens 10 is a piano-convex singlet having a curved surface 1 and a plano-surface 3, which is the surface opposite to the curved surface 1 and has a Fresnel zone pattern. The curved surface 1 and the plano-surface 3 are perpendicular to the optical axis of the lens 10. A refractive lens is made of a material having a high index of refraction of at least 1.65.
However, since the hybrid lens 10 shown in FIG. 1 is made of a material having a low index of refraction and a low dispersion, the weight and the size of the lens 10 are great. Further, since a diffractive surface is formed in a surface of the lens 10 opposite to a disc, the lens 10 is easily worn due to the contact with the disc and the performance of the lens 10 is also decreased due to a pollution source.